This invention relates to non-fibrillatable fluoropolymers and more particularly, to sprayable powders thereof.
Fluoropolymer resin having properties such as good chemical resistance, excellent release, good heat resistance and electrical insulation are desirable in a number of applications. Fluoropolymer powders which are melt-flowable have been found useful in coating cookware articles such as fry pans, sauce pans, rice cookers, grills, and bakeware as well as numerous industrial applications such as fuser rolls or belts for copiers and printers, and chemical processing reactors. One of the advantages for applying powder coatings in lieu of liquid coatings is the absence of volatile organic solvents that present environmental concerns and necessitate expensive remediation procedures.
Canadian patent 1,248,292 (Buckmaster et al.) discloses a conventional process for preparation of the dry powder, in which the aqueous polymerization dispersion of the fluoropolymer is chemically treated with a gelling agent while being agitated and then treated with a water immiscible organic liquid (usually a halocarbon). This treatment causes the primary fluoropolymer particles, generally of average particle-size of 0.1 to 0.5 micrometers, to coagulate (agglomerate) into granules generally having an average particle size of 200 to 3000 micrometers over a lengthy period of agitation. This process is generally known as xe2x80x9csolvent-aided coagulationxe2x80x9d. To render the granules essentially non-friable, they are heat-hardened at temperatures close to the melting temperature of the fluoropolymer.
While the resultant granules are valuable for use in such processes as rotocasting and rotolining, they are less useful for coating applications. The granules are too large to make uniform coatings. To be useful, the granules are comminuted to smaller particle sizes, which increases cost and tends to fibrillate the heat hardened granules and thereby cause plugging of spraying equipment used for applying the coating. Comminution of non-heat hardened granules produces excessive fines (particles less than 1 micrometer) which have poor handling and coating characteristics.
Another method for producing fluoropolymer powder is disclosed in U.S. Pat. No. 3,953,412 (Saito). This reference discloses that an aqueous dispersion of tetrafluoroethylene polymer is atomized into a gas at a temperature above the melting point of tetrafluoroethylene polymer, whereby the dispersed solids are coalesced and sintered to produce sintered micro-powder with an average particle size of 100 microns or less. Sintered spherical particles of the Saito invention are described as useful as lubricants and for molding applications. These nearly spherical particles are suited for processing by rotomolding or casting. But because these particles have been sintered (that is heat hardened at temperatures close to the melting point of the polymer) it is difficult to further tailor the particle size, such as by grinding, to produce particles for certain spraying applications, as the particles are essentially non-friable.
Attempts have been made to produce blended fluoropolymer powder coatings with multiple components, such as fluoropolymer blended with polymer binder discussed in U.S. Pat. No. 5,093,403 (Rau et al.). In ""403, the particle dimensions of the formulation components are determined by grinding or milling capabilities and limit the composition uniformity of coatings fabricated from the blend. Such blends also exhibit segregation in shipping, processing and recovery of used powder. When using a powder mixture for electrostatic spraying, the components of the mixture tend to segregate. The segregation can produce a variable powder coating composition when the powder is sprayed on a substrate. Another notable problem occurs in electrostatic spray coating in conjunction with the sprayed powder that does not adhere to the target substrate, known as xe2x80x9coversprayxe2x80x9d. Segregation in the overspray prevents direct recycle to the spraying operation.
There remains a need for a uniform composition of sprayable powder of non-fibrillatable fluoropolymer optionally blended with other components that can be produced, in the absence of a processing step involving halocarbon solvents and/or without the necessity of sintering the particles.
In a first embodiment, this invention provides for sprayable powder comprising friable granules of agglomerated primary particles of non-fibrillatable fluoropolymer and at least one other component, the powder having a bulk density of at least 20 g/100 cc and average particle size of 5 to 100 micrometers. The sprayable powder is preferably free of water immiscible liquid, and more preferably free of halocarbon liquid. Included among the other components are inorganic fillers, pigments, high temperature resistant polymer binders.
In another embodiment, the invention provides for a sprayable powder comprising friable granules of agglomerated primary particles of a first non-fibrillatable fluoropolymer and at least one other non-fibrillatable fluoropolymer component.
The invention further provides for a process for preparing the sprayable powder by spray drying a liquid dispersion of primary particles of non-fibrillatable fluoropolymer and at least one other component to obtain friable granules. The dispersion is preferably free of water immiscible liquid and more preferably free of halocarbon liquid. In a preferred embodiment, the process includes preparing sprayable powder of non-fibrillatable fluoropolymer and, optionally, at least one other component, and densifying the granules of agglomerated primary particles that result from spray drying. Densifying may be carried out by mechanical compaction or by contacting the granules with a heated gas to form a fluidized bed. Optional steps of comminution and heat treatment may be employed to achieve a desired bulk density and particle size for specific applications.
Another embodiment of the invention provides for sprayable powder of non-fibrillatable fluoropolymer comprising a uniform, non-segregatable mixture of friable granules of agglomerated primary particles of the fluoropolymer and at least one other component wherein the powders are melt-processible.
The objects of the present invention may be obtained without sintering of the particles or the use of halocarbon solvents.